Optical fibers are key components in modern telecommunication systems. Basically, an optical fiber is a thin strand of glass capable of transmitting optical signals containing a large amount of information over long distances with very low loss. In its simplest form, it is a small diameter waveguide comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Typical optical fibers are made of high purity silica with minor concentrations of dopants to control the index of refraction.
Optical gratings are important elements for selectively controlling specific wavelengths of light within optical systems such as optical communication systems. Such gratings include Bragg gratings and long period gratings. Gratings typically comprise a body of material and a plurality of substantially equally spaced optical grating elements such as index perturbations, slits or grooves. The ability to dynamically modify these gratings would both be highly useful.
A typical Bragg grating comprises a length of optical waveguide, such as optical fiber, including a plurality of perturbations in the index of refraction substantially equally spaced along the waveguide length. These perturbations selectively reflect light of wavelength .lambda. equal to twice the spacing .LAMBDA. between successive perturbations times the effective refractive index, i.e. .lambda.=2n.sub.eff.LAMBDA., where .lambda. is the vacuum wavelength and n.sub.eff is the effective refractive index of the propagating mode. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings have found use in a variety of applications including filtering, adding and dropping signal channels, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and compensation for waveguide dispersion.
Waveguide Bragg gratings are conventionally fabricated by doping a waveguide core with one or more dopants sensitive to ultraviolet light, e.g., germanium or phosphorous, and exposing the waveguide at spatially periodic intervals to a high intensity ultraviolet light source, e.g., an excimer laser. The ultraviolet light interacts with the photosensitive dopant to produce long-term perturbations in the local index of refraction. The appropriate periodic spacing of perturbations to achieve a conventional grating can be obtained by use of a physical mask, a phase mask, or a pair of interfering beams.
The performance of high speed WDM lightwave systems depends critically on the details of the system design and particularly on the level of in-line dispersion and dispersion slope compensation as well as nonlinear effects occurring in the dispersion compensated fiber (DCF). In such systems small variations in optical power, due for example to imperfect gain flattening, can result in additional nonlinear phase shift that can modify the optimal dispersion map of the system. This problem is exacerbated by a reduced dispersion budget associated with imperfect dispersion slope compensation over a wide bandwidth of operation. For example, in a typical system operating with approximately 40 nm of bandwidth, and with an uncompensated dispersion slope of 0.05 ps/nm.sup.2.km, the accumulated divergence in the dispersion (assuming approximately 60% compensation in dispersion compensating fiber), is thus approximately 1.2 ps/nm.km. The corresponding dispersion budget is typically taken as twice this value giving 2.4 ps/nm.km. It follows that the maximum transmission distance (L) that can be achieved before incurring a significant penalty, is given L&lt;104,000/(B).sup.2.vertline.D.vertline. (Gb/s).sup.2 ps/nm (1) where B is the channel rate and D the dispersion of the fiber, is 32 km for a 40 Gbit/s system and 512 km for a 10 Gbit/s system. Thus while many anticipate the need for dynamic dispersion compensation at 40 Gbits/s clearly some 10 Gbits/s systems will benefit from this as well. Dispersion compensating devices that provide dynamically adjustable dispersion and that can be incorporated directly into the fiber are thus particularly attractive.
In conventional Bragg gratings the dispersions and reflective properties are static. Each grating selectively reflects only light in a narrow bandwidth centered around m.lambda.=2n.sub.eff.LAMBDA., where m=1,2,3 . . . is the order of the grating. However for many applications, such as dispersion compensation, it is desirable to have gratings which can be controllably altered in bandwidth and/or dispersion.
Chirped waveguide gratings are promising elements for compensating system dispersion. Chirped Bragg gratings operated in reflection can provide necessary dispersion. However most waveguide Bragg gratings are not readily adjustable, meaning that they are not easily tuned or adjusted after fabrication, non-linear chirping is difficult to achieve, and currently available devices are not well suited for small size, low power applications.
One attempt to induce dynamically adjustable chirp on a waveguide Bragg grating involves setting the waveguide in a groove in an elongated plate and imposing a temperature gradient on the plate. See U.S. Pat. No. 5,671,307 issued to J. Lauzon et al, on Sep. 23, 1997. This arrangement, however, has a number of shortcomings:
(a) It is bulky and not easily compatible with existing packaging technologies. PA1 (b) Because of this bulkiness the device is power inefficient, and adds a latency to the dynamic response of the device. Power is required to heat the elongated plate, which can be much larger than the fiber itself. PA1 (c) There is significant heat transfer to the surrounding environment because heat must be continuously supplied to support constant heat flow along the plate. PA1 (d) It is difficult to prescribe complex/arbitrary temperature gradients and thus complex dispersions. The heat sink and heat source allow for only a linear temperature gradient. In many cases, however, nonlinear temperature gradients are required for producing nonlinear chirps.
Controllable chirping is also potentially useful for long-period gratings. Long-period fiber grating devices provide wavelength dependent loss and may be used for spectral shaping. A long-period grating couples optical power between two copropagating modes with very low back reflections. It typically comprises a length of optical waveguide wherein a plurality of refractive index perturbations are spaced along the waveguide by a periodic distance .LAMBDA.' which is large compared to the wavelength .lambda. of the transmitted light. In contrast with conventional Bragg gratings, long-period gratings use a periodic spacing .LAMBDA.' which is typically at least 10 times larger than the transmitted wavelength, i.e. .LAMBDA.'.gtoreq.10.lambda.. Typically .LAMBDA.' is in the range 15-1500 micrometers, and the width of a perturbation is in the range 1/5 .LAMBDA.' to 4/5 .LAMBDA.'. In some applications, such as chirped gratings, the spacing .LAMBDA.' can vary along the length of the grating.
Long-period gratings are particularly useful for equalizing amplifier gain at different wavelengths of an optical communications system. See, for example, U.S. Pat. No. 5,430,817 issued to A. M. Vengsarkar on Jul. 4, 1995.
Conventional long-period gratings are permanent and narrowband. Each long-period grating with a given periodicity (.LAMBDA.') selectively filters light in a narrow bandwidth centered around the peak wavelength of coupling, .lambda..sub.p. This wavelength is determined by .lambda..sub.p =(n.sub.g -n.sub.ng). .LAMBDA.'where n.sub.g and n.sub.ng are the effective indices of the core and the cladding modes, respectively. The value of n.sub.g is dependent on the core and cladding refractive index while n.sub.ng is dependent on core, cladding and ambient indices. Adjustable chirp long-period gratings could be highly useful for compensating gain spectrum change.
Accordingly there is a need for improved optical waveguide grating devices with adjustable chirp.